a. Cedric used an equal sign (=) in the equation, indicating equality between two expressions, making it an equation. This means that the amount of money Myron had before spending on the video game is equal to the amount he had left after spending. So, it's an equation representing a balance or equality between the two quantities.
b. Cedric subtracted m from 20 because Myron spent money, causing his balance to decrease. Adding m to 20 would imply an increase in his balance, which goes against the problem's context.